The present invention relates generally to a steam turbine power plant and, more particularly, to a steam turbine power plant which incorporates a liquid-to-liquid feedwater heater in conjunction with a plurality of vapor-to-liquid feedwater heaters.
In a steam power plant, a plurality of feedwater heaters are usually employed to increase the temperature of a condensate taken from a condenser within the steam power system prior to reintroducing that condensate into a steam generator. By increasing the temperature of the condensate before it is reintroduced into the steam generator, the overall efficiency of the power plant is improved. It has been the practice in the art to heat the condensate in one or more feedwater heaters which use steam taken from extraction ports of a high pressure turbine element. Also, low pressure feedwater heaters can be employed in series with high pressure feedwater heaters between the condenser and the steam generator in order to provide gradual step increases in the temperature of the feedwater as it passes from the condenser to the steam generator. The steam which is used to raise the temperature of the feedwater is taken from the low and high pressure turbine elements through extraction ports. It is known in the art to also utilize the condensed throttle steam from the drain of a reheater such as a moisture separator-reheater.
When a plurality of feedwater heaters is used, each feedwater heater has a vapor inlet which is connected in fluid communication with an extraction port of one of the steam turbine elements. After this extracted steam passes in thermal communication with the feedwater within the feedwater heater, it condenses and is removed from the feedwater heater through a condensate outlet. It is known in the art to connect the condensate outlet of one feedwater heater to an inlet of a lower pressure feedwater heater within the system. This technique allows this condensate to flash within the lower pressured feedwater heater and, due to its higher temperature, help to increase the temperature of the feedwater passing through that lower pressure feedwater heater. When a plurality of feedwater heaters are connected in series, it is common practice for some of the feedwater heaters, except the one which is at the lowest pressure, to be cascaded backward, in a direction opposite to that of the feedwater flow, toward the condenser in this manner. Eventually, this condensate is introduced into the feedwater heater which is operating at the lowest pressure and, after raising the temperature of the feedwater passing through this low pressure feedwater heater, its condensate is introduced into an inlet port of a condenser. By utilizing systems of this type, much of the heat from the steam which is removed from the turbine extraction ports can be effectively transferred to the feedwater prior to its entry into the steam generator.
Another technique known to those skilled in the art is to introduce the condensate from one of the high pressure feedwater heaters directly into the stream of feedwater as it flows toward the steam generator. When this technique is utilized, this condensate from a high pressure feedwater heater is generally introduced upstream from a feedwater pump.
U.S. Pat. No. 3,973,402, which issued to Silvestri on Aug. 10, 1976, describes a pressure increasing ejector element which is disposed in an extraction line between a high pressure turbine element and a feedwater heater. The purpose of this ejector element is to increase the pressure at which the extraction steam is introduced into the feedwater heater by utilizing high pressure fluid from a reheater drain. U.S. Pat. No. 4,336,105 issued to Silvestri on June 22, 1982, describes a nuclear power plant steam system which utilizes steam from two extraction ports of a steam turbine in order to heat water in at least two feedwater heaters.
Feedwater heaters of many types are known to those skilled in the art. U.S. Pat. No. 3,795,273 which issued to Brigida et al. on Mar. 5, 1974 describes a feedwater heater designed for use in a power plant system in which steam from another unit in the system is introduced into the shell of the heater and the feedwater heater discharges condensate to another unit in the system. The Brigida patent describes a feedwater heater in which feedwater is circulated through tubes in the shell in thermal communication with the steam which is thereby condensed. Another portion of the steam is directed to an area of the shell where it warms the condensate to a degree that maintains the condensate at or near its saturation temperature.
U.S. Pat. No. 3,885,621 which issued to Slebodnick on May 27, 1975 discusses a vertically disposed feedwater heater which has an upper portion which is sealed by a water seal, thereby forming a vent condenser. This type of feedwater heater further comprises a plurality of telescoping skirts and a collar which cooperate to form this water seal.
U.S. Pat. No. 3,938,588 which issued to Coit et al. on Feb. 17, 1976 describes a feedwater heater which has a condensate inlet and a flow distributor which are cooperatively associated, a plurality of U-shaped tubes, a vent condenser portion and a centrally disposed trough within its tube bundle. The purpose of the Coit patent is to provide a feedwater heater which deaerates the condensate fluid.
U.S. Pat. No. 4,136,734 which issued to Sasaki et al. on Jan. 3, 1979 discloses a feedwater heater which has a hot steam inlet for introducing high temperature steam, such as a bleed from a steam turbine extraction port. It comprises a generally cylindrical body which is further provided with a condenser outlet for discharging steam condensate out of the feedwater heater unit. U.S. Pat. No. 4,207,842 which issued to Kehihofer on June 17, 1980 discloses a mixed flow feedwater heater which has a regulating device and a feedwater tank having a deaerating dome.
In typical nuclear power cycles, moisture separator-reheaters are used at the inlet portion of a low pressure turbine element in order to improve the cycle efficiency of the system and to reduce blade erosion which could be caused by entrained moisture in the steam. For lower pressure nuclear power plants which have essentially dry and saturated or low superheat steam at the throttle, this moisture separation occurs at the high pressure turbine exhaust. A moisture separator-reheater restores the steam to a dry and saturated condition. The moisture which is present in the steam entering the moisture separator-reheater is then removed from the steam flow and is conducted to a feedwater heater. Generally, this moisture separator-reheater drain water is introduced to the feedwater heater which is connected in fluid communication with the extraction port of a high turbine element. In some cases, it has been found necessary to cascade the drain water from the moisture separator-reheater to a lower pressure feedwater heater in order to insure positive separator drainage. U.S. Pat. No. 4,206,802 which issued to Reed et al. on June 10, 1980 discloses a moisture separator-reheater which incorporates a plurality of tube bundles which receive high pressure saturated steam therein. Steam which is to be reheated is passed in heat exchange relationship with the tubes of the first and second reheater tube bundles after first being dried by the panels of a moisture separator.
In nuclear power plants which utilize oncethrough steam generators, units which utilize pumped forward drains which have a water impurity inflow, such as units with demineralizers or with persistent condenser leakage, the impurity concentration of inlet steam and feedwater increases by the value of inflow for every circulation cycle and eventually reaches a limiting value. This problem occurs because most of the impurities from the high pressure turbine steam are concentrated in the separator drains. This concentration of impurities occurs because the solubility of impurities in water is several orders of magnitude higher than their solubility in steam. Furthermore, in the transition of steam with impurities from the dry to the wet phase in the high pressure turbine element, most of the water droplets form on impurity precipitates as nucleation centers and many impurities, such as sodium salts, are hygroscopic and therefore absorb moisture. One potential solution to this concentration problem which has been considered by those skilled in the art is to cascade all of the heater drains toward lower pressure feedwater heaters and eventually back to the condenser. Although this possible solution ameliorates the impurity concentration problem, it has a significant negative impact on cycle efficiency by increasing the heat rate by as much as 0.43%.
The present invention utilizes a heat exchanger which is interspersed between the feedwater heater which is connected to a high pressure extraction port and the next lower pressure feedwater heater. The heat rate in this type of configuration is improved by approximately 0.31% as compared to the alternative which cascades the condensate from all of the feedwater heaters to lower pressured feedwater heaters and eventually to the condenser. The liquid-to-liquid heat exchanger of the present invention receives the water from the moisture separator drain of the moisture separator-reheater and then cascades it to the next lower pressure feedwater heater. The heat exchanger utilized in the present invention can be similar, in operation, to a drain cooler. Although the present invention can have a heat rate which is poorer than a system which incorporates total reverse cascading, it is not subject to the concentration of impurities which would otherwise be present. The present invention reduces the impurity concentrating mechanism of other alternative configurations while minimizing the heat rate loss as compared to other alternatives. Furthermore, the present invention avoids the necessity of increasing the condensate flow capability of the low pressure feedwater heaters which would otherwise be necessitated if all feedwater heaters were cascaded back toward lower pressure feedwater heaters. This characteristic is especially important in situations where an existing power plant is to be retrofitted. If all of the feedwater heaters of the plant were to be reconnected in order to convert to a totally cascading cycle, the lower pressure feedwater heaters would experience a condensate flow increase of approximately 45%. It is highly probable that these lower pressure feedwater heaters could not be operated with this additional condensate flow. If the present invention is utilized to retrofit an existing power plant, the lower pressure feedwater heater condensate flow would increase only by approximately 16%.
The water received from the separator drain of a moisture separator-reheater will typically have a comparatively high contaminant level as compared to the steam received from the extraction ports of the high and low pressure turbine elements. This drain water from the moisture separator-reheater would not pass through a condensate demineralizer, which would be located in the lowest temperature end of the condensate stream, to purify it. During each pass of the water through the condenser and steam generator system, there would be an increase in the contaminant level in the condensate which enters the steam generator and the steam which leaves the steam generator. Furthermore, the volume of drain water being cascaded through the low pressure feedwater heaters may be beyond the capacity of these heaters to function properly. This condition may cause flooding of the low pressure feedwater heaters and any flooding of a feedwater heater will impair its ability to transfer heat, resulting in an increase of the heat rate.
A typical embodiment of the present invention would incorporate three low pressure feedwater heaters in which two of the feedwater heaters are cascaded backward so that their condensate flows into the next lower feedwater heater. The lowest pressure feedwater heater would then pass its condensate to the condenser. A water-to-water heat exchanger would be connected to the separator drain of a moisture separator-reheater and the drain water outlet of this water-to-water heat exchanger would be connected in fluid communication to an inlet of the highest of the three low pressure feedwater heaters. The feedwater would pass serially through the three low pressure feedwater heaters and then through the water-to-water feedwater heater. After passing through the water-to-water feedwater heater, the feedwater would then pass serially through two high pressure feedwater heaters in which the highest pressure feedwater heater condensate would be introduced into the next highest pressure feedwater heater. The condensate from this feedwater heater would then be introduced directly into the feedwater stream.